The present invention relates generally to laser systems, and more particularly to master-oscillator power-amplifier laser systems which include distortion-compensating phase conjugate mirrors.
A high-power laser beam can be obtained by using a master-oscillator power-amplifier ("MOPA") laser system having an oscillator to provide a laser beam and a power amplifier to amplify the beam. However, such an amplifier introduces distortion as it amplifies, and it is necessary to compensate for this distortion in order to provide an output beam of acceptably high quality. One way of compensating for such distortion is to use phase conjugation means such as a phase conjugate mirror.
A phase conjugate mirror receives an input laser beam and provides a "reflected" beam which is a conjugate of the received beam. If the received beam has been distorted by propagation through a distorting medium en route to the phase conjugate mirror, and if the "reflected" conjugate beam is then directed back through the distorting medium, a substantial portion of the distortion vanishes and the conjugate beam emerges, essentially distortion-free, from the distorting medium.
A high-power laser system which uses phase conjugation to compensate for amplifier distortion includes a laser oscillator to provide a laser beam, a power amplifier, a beam splitter to reflect a portion of the beam from the oscillator into the amplifier, and a phase conjugate mirror to receive the amplified beam from the amplifier and to reflect a conjugate beam back through the amplifier. As the conjugate beam goes back through the amplifier, it is further amplified and then passed through the beam splitter to provide a high-power output laser beam. Any distortion introduced into the beam during amplification is removed when the conjugate beam goes back through the amplifier, and accordingly the output laser beam is virtually distortion-free.
The laser oscillator which provides the input beam comprises an amplifying medium disposed between a reflector and an opposing partial reflector. The input beam emerges from the partial reflector and propagates from there toward the beam splitter.
The phase conjugate mirror may comprise a stimulated brillouin scattering ("SBS") mirror. Unlike other kinds of phase conjugate mirrors, SBS mirrors require no "pumps" or other external power supplies and hence are relatively efficient. However, a typical SBS mirror reflects no more than 60% to 80% of the power it receives, and it is therefore necessary to use a very high gain amplifier to make up for this loss and provide the desired high-power output beam. For these reasons, an amplifier having a gain of 200 or more may be required.
Although a MOPA laser system as described, including a high-gain amplifier that offsets losses in the SBS mirror, can provide a well-compensated high-power laser beam, such a system can become unstable. This instability results from the fact that the beam splitter does not pass all of the amplified conjugate beam through to the output. Instead, a portion of this amplified beam is reflected back toward the partial reflector of the oscillator, from whence the beam is reflected back through the splitter toward the amplifier for a second round trip through the amplifier and the SBS mirror. In other words, the combination of the amplifier, the SBS mirror, and the partial reflector constitutes an oscillator. If the round-trip gain through this combination exceeds unity, oscillation occurs, and if the amplifier gain is high enough this unwanted oscillation not only prevents the system from operating as desired but can physically destroy some of the components.
This unwanted oscillation can theoretically be prevented by means of polarizing devices. In one such polarizing device, the input beam is polarized by Brewster windows in the laser oscillator and the polarized beam is then oriented to a desired orientation by a half-wave plate. A polarization-separating beam splitter receives the polarized input beam from the half-wave plate and directs it into the amplifier. A quarter-wave plate between the amplifier and the SBS mirror rotates the polarization of the beam as it passes between the amplifier and the SBS mirror such that, after the return pass through the amplifier, the amplified conjugate beam is orthogonally polarized with respect to the input beam. The polarization-separating beam splitter then directs the amplified beam to the output.
If the polarization of the amplified beam is kept perfectly orthogonal with respect to the polarization of the input beam, no undesired oscillation can occur because only a very small percentage of the amplified beam gets reflected back toward the partial reflector of the oscillator when high quality polarizers are used. However, even a very slight birefringence in the amplifier or in any other system element can depolarize the beam enough to result in oscillation. A depolarization as small as 1% in a system having an amplifier with a gain of 200 can lead to unwanted oscillation.
Another problem is presented by amplified spontaneous emissions ("ASE") originating in and amplified by the amplifier which are reflected by the SBS mirror and then, in passing back through the amplifier, are further amplified. Since the ASE are initially unpolarized, about half of the ASE will be directed by the beam splitter toward the oscillator. Any ASE entering the oscillator can degrade the performance of the oscillator and may even quench the oscillator entirely.
It will be apparent from the foregoing that there is a need for a means to prevent unwanted oscillation in a MOPA laser system having a distortion-compensating phase conjugate mirror, and to prevent any ASE which may be present in the output of such a system from degrading the system oscillator.